1. Field of the Invention
This invention relates to forming microstructures and more particularly to forming microstructures by electrodeposition.
2. Brief Description of the Prior Art
Rapid growth in the area of microelectromechanical systems (MEMS) has created a demand for 3-dimensional micron-scale components. Microelectromechanical devices comprise structures of generally conventional shape and function, e.g., beams, posts levers, wheels, and the like, but of a size that is microscopic. Typically, the overall sizes of such devices are no more than a few millimeters in any dimension. The practical lower limit on the size of such devices is uncertain, but entire devices only a few micrometers across are envisaged. As the general name implies, MEMS often incorporate electrical elements as sensors and/or actuators.
Evidently, MEMS are not readily capable of being manufactured by conventional machining techniques, which are too coarse for the fabrication and assembly of the miniature and delicate elements that constitute such devices. Typically, the techniques that have proved successful in fabricating microscopic electrical devices such as transistors, integrated circuits, microprocessors, and the like, have been adapted to the construction of MEMS. Accordingly, microlithographic methods have been used to form shaped structures on substrates. The adaptation of semiconductor manufacturing techniques has also been favored because silicon has been found to be a useful material for making MEMS. However, structures of metal have also assumed importance for construction of MEMS. By such procedures, successive stages of applying a resist layer, patterning the layer by imaging and developing, and forming a structure corresponding to the pattern have been used. The structures may be formed either by etching a substrate according to the patterned resist layer or by depositing a metal in the developed pattern of the resist to form a pattern in relief on the substrate surface. Conventional deposition techniques, such as chemical vapor deposition, electrodeposition, and the like can be used. Successive stages of patterned deposition and etching can result in a 3-dimensional mechanical structure.
However, current techniques involve multiple steps, using many expensive and sometimes hazardous materials, and tend to generate large volumes of waste materials relative to the numbers of devices produced. As a result of these disadvantages, manufacture of MEMS is currently time-consuming and expensive.
A particular problem encountered in MEMS manufacture, which is not so often experienced in fabrication of semiconductor devices is the need to provide free-standing posts or similar structures extending above a supporting substrate surface. Such structures may require vertical dimensions and aspect ratios greater than those commonly demanded in the fabrication of electrical semiconductor devices.
Accordingly, a need has continued to exist for improved methods of constructing microelectromechanical devices, and, in particular for fabricating free-standing structures of relatively great vertical dimension.
The problem of manufacturing microscopic mechanical elements extending vertically from a supporting substrate surface has now been alleviated by the method of this invention. According to the invention structures of metal or other material that can be electrolytically deposited are formed directly by a method wherein a low-relief base is formed on a substrate surface by any conventional means, and electrolytically depositable material, e.g., metal, is preferentially deposited on the upper surface of the base to produce a vertically-extending structure. The metal, or the like, is preferentially deposited on the upper surface or tip of the base by contacting the base and a counterelectrode with an electroplating bath and passing an electric current is passed between the substrate and counterelectrode, wherein the substrate is predominantly cathodic with respect the counterelectrode. In a first step the electrolytic environment at the substrate surface is maintained as a microprofile, whereby metal is deposited preferentially at the upper edge or tip of the base until the structure has been increased in height, and, in a second step, the electrolytic environment at the substrate surface is maintained as a macroprofile to continue the deposition of metal at the upper edge or tip of the structure until the desired relief is obtained.
Accordingly, it is an object of the invention to provide a method for manufacturing MEMS.
A further object is to provide a method for electro-chemically producing free-standing microscopic structures on a substrate surface.
A further object is to provide a method for converting a microscopic low-relief pattern on a substrate surface into a pattern of higher relief.
A further object is to provide an electrochemical method for increasing the height of a free-standing structure on a substrate surface.
A further object is to provide a method for electrochemically producing posts and the like suitable for use in constructing MEMS.
Further objects of the invention will be apparent from the description of the invention that follows.